Photovoltaic technology is potentially suitable to satisfy the energy need of a growing world population being confronted with a simultaneously decreasing availability of resources. Lifecycle assessment of the manufacturing process of the widely-used silicon solar cells reveals that their use is largely offset by the required energy expenditure. The high energy expenditure for the manufacture of silicon-based solar cells influences the overall production costs and thus inhibits the spread of an otherwise clean energy source at least in unsubsidized markets.
A concept for reducing the production costs of solar cells, which is already commercially successful, uses semiconductors with very high absorption coefficients. Thus, the active layers can be designed to be of low thickness and are therefore cheaper to produce. The absorption coefficients of cadmium telluride or of CuInS(Se) (CIS) are about 100 times larger than the absorption coefficient of silicon. Therefore, the thickness of the absorption layer of thin-film solar cells can be reduced by about the same factor with respect to conventional silicon-based solar cells.
However, the actual cost saving with regard to the finished solar modules merely is about 10 to 20%, because the current thin film technology requires high investments in high-vacuum evaporation plants and in particular, because chemical vapor deposition is a slow process.
In contrast thereto, printing processes for paper and sheets are matured, efficient and fast technologies. Therefore, numerous attempts have been made to adopt the technical expertise from said printing processes for the manufacturing of thin-film solar cells and semiconductor cells.
Printing processes have been successfully used for producing organic solar cells or solar cells basing on colorants. However, such dye-sensitized solar cells suffer from low light resistance and therefore have a short lifetime when being exposed to solar radiation. Consequently, only inorganic semiconductors can serve for the long-term generation of energy.
In order to use established printing processes for the manufacture of such photovoltaic cells based on inorganic semiconductor thin films, the development of novel inks is necessary. Since the highest energy yields can be achieved with CuInGaSe (CIGS) solar cells (apart from the highly toxic GaAs), most attempts focus on this material. Numerous processes have been described wherein inks for producing p-layers of CIS- or CIGS-type solar cells are used. Therein, the n-layer is still produced by chemical vapor deposition or chemical bath deposition. Since CIS/CIGS and CZTS are not soluble in currently known solvents, or only soluble to an extent that renders the printing process impractical, in the above-mentioned printing processes, suspensions of solid CIS particles are used as inks for printing; cf. for example U.S. Pat. No. 7,663,057 B2.
However, if said particles are too large (in the μm range), they sediment rapidly. A severe disadvantage of these processes is that the contact surface between the metallic conductor and the p-layer decreases with increasing particle size, having a negative impact on the conductivity between those layers. Accordingly, the conductivity within the p-layer is substantially reduced. When using such particle inks, the efficiency of the photon yield is therefore restricted. Attempts have been made to reduce the particle size, even down to the range of quantum dots. However, quantum mechanical effects lead to complications when reducing the particle size below a certain limit. For instance, the maximum theoretical quantum yield is influenced by the band gap, which in turn becomes dependent on the particle size when the latter becomes increasingly small. In addition, particles with very small sizes need to be stabilized chemically. Accordingly, if use is made of stabilizers that are strongly attached to the particle surface it becomes increasingly difficult to obtain the necessary purity of the desired semiconductor material.
In order to overcome the problems associated with inks using semiconductor particles, concepts have been developed where instead of inks containing said particles, liquid precursor solutions are used, wherein the desired semiconductor material is formed by an in situ reaction after the printing step. For the manufacturing of CIGS or CZTS (copper, zink, tin sulfide/selenide) solar cells the easiest possible approach would have been the use of soluble salts of the metal elements and to print such a solution onto a solar cell substrate. Then, in a separate process, sulfur and/or selenium could have been introduced by applying for example a (NH4)2S or a (NH4)2Se solution. However, this approach has turned out to be impractical for a plurality of reasons. A better approach is to use urea or thiocarbamide or selenocarbamide as sulfur/selenium source instead of free sulfide or selenide ions, and a liquid precursor solution containing all elements necessary for the in situ reaction may thus be obtained. The urea or thiocarbamide derivatives are stable up to a certain temperature so that, after printing the precursor solution, the desired compounds, e.g. CIS or CZTS, may be formed by a subsequent heating step. This concept is termed spray-pyrolysis and well-known in the field. Although a lot of research efforts have been made, the maximum achievable energy yields when using such spray pyrolysis are low. One reason for this resides in problems associated with the in situ reaction: By-products that are formed during the in situ reaction do not evaporate completely and thus reduce the purity of the desired product. In addition, crystallization of the product is impaired because the anions of the soluble metal cation salts typically form ammonium chloride which has an evaporation point above the film forming temperature and can destroy the layer integrity. A major problem associated with precursor solutions for in situ reaction are impurities formed by precursor ligands containing at least one of carbon and/or nitrogen and/or phosphorous and/or oxygen. Such impurities can substantially reduce the efficiency of semiconductors.
U.S. Pat. No. 5,714,391 describes vacuum-free vapor deposition of sulfide thin films by thermal decomposition of precursor compounds. The precursor compounds are volatized at temperatures above 240° C. The thickness of the thus produced films is preferably less then 700 nm, since the carbon content starts to increase dramatically at a film thickness of about 700 nm. Therefore, these films are not suitable to form the p-type layer of a solar cell where a minimum thickness in the μm-range, i.e. at least 1 μm, is required.
However, pyrolysis of compounds with such a high thermal decomposition temperature usually leads to a high concentration of impurities in the product, especially carbon and carbonaceous compounds. In addition, merely the synthesis of binary compounds (e.g. CuS, Cu2S, CdS, etc.) are described in the two above-mentioned documents, without addressing the synthesis of compounds containing more than two elements, such as CIS, CIGS, CZTS and the like.